The present invention relates to a process for producing titanium dioxide of a pigment grade, parts of the process, and the product of the process. In particular, the present invention relates to the processing of titaniferous ore, especially ilmenite ore to TiO2 pigment. The process includes a novel combination of operational steps to economically produce a high quality grade titanium dioxide pigment.
Titanium dioxide is considered the principal white pigment of commerce. It has exceptionally high refractive index, negligible color and is quite inert. Titanium dioxide may be present in either of two predominant forms, anatase or rutile. For the majority of commercial applications, rutile is the desired form.
There are two main forms of titanium ore available. One is mineral rutile, which is composed of greater than about 90% to 95% titanium dioxide. The other is ilmenite (generally having the formula FeOTiO2 or FeTiO3), which contains from about 45% to about 65% titanium dioxide. It is known to upgrade the ilmenite to titania slag, which contains about 85% titanium dioxide and about 10% iron oxide.
There are two main processes for making raw pigmentary titanium dioxide, the sulfate process and the chloride process. The sulfate process relies on ilmenite or titania slag as the raw material. Generally, the ilmenite is digested in concentrated sulfuric acid, iron sulfate is separated after cooling, and titanium hydrolysate e.g. hydrated TiOSO4 is precipitated by addition of water following special procedures. The precipitate is calcined to form TiO2 with the desired properties. The advantage to this process is that ilmenite may be used as the starting ore, which is relatively plentiful, particularly when compared to the diminishing reserves of rutile. The disadvantages of this process include a necessarily high input of energy, expensive and complicated equipment, long processing times, and undesirably large volumes of acidic liquid wastes, containing iron sulfate.
The chloride process relies on chlorination of a low iron titanium ore followed by the gas-phase oxidation of TiCl4. One disadvantage to this process is that the starting material, rutile, is becoming scarce. In addition, direct chlorination of ilmenite is generally not economical because the ilmenite contains a substantial amount of iron, which converts a substantial amount of the chlorine to iron chloride, making it unavailable to chlorinate the titanium.
Processes exist to remove iron from ilmenite and similar ores, and to produce synthetic rutile, which can be used in the chloride process. For example, the Becher process (U.S. Pat. No. 3,502,460), the Benilite process (U.S. Pat. No. 3,967,954) and the Murso process are known. These processes consist of pretreatment steps followed by partial leaching in hydrochloric acid. The procedure involves several stages and is expensive, particularly since the synthetic rutile product is impure and must be further treated by the chlorination process.
U.S. Pat. No. 3,903,239 teaches a process where the titanium as well as the iron contained in ilmenite ore is dissolved in concentrated hydrochloric acid and the iron is subsequently reduced and precipitated as ferrous chloride. The titanium is precipitated by adding water to the solution after separation of iron chloride. To limit the amount of water to be added and to keep the total amount of solution to be regenerated small, the amount of acid used in the leaching process is kept as low as possible. Also, to avoid hydrolysis of TiO2 during leaching with this limited excess of acid, the temperature is kept low and the leaching time is on the order of several days. The process of the present invention also involves dissolution of both titanium and iron, and precipitation of iron chloride, but the following description will show the different purpose of the leaching and regeneration steps and the greater advantages resulting from the process of the present invention.
Two significant advantages of the present process over that taught in U.S. Pat. No. 3,903,239, include the use of HCl gas to supplement the acid consumed during the leaching process and the use of solvent extraction to purify the titanium solution. The use of HCl gas enhances leaching rates, increases the amount of ilmenite dissolved and enhances the product quality. The use of solvent extraction produces a hydrolyzed TiO2 product with a much lower impurity level.
While U.S. Pat. No. 3,903,239 teaches the production of a titanium dioxide product with an iron contamination ranging between 100 and 200 ppm Fe, the present invention produces a SX raffinate containing 100 gpI Ti with only 1 ppm Fe. This results in a final titanium dioxide pigment product that contains only around 6 ppm Fe.
The market for common titanium dioxide pigment products generally requires a maximum iron specification of no more than 30-50 ppm Fe. Therefore, the process according to that disclosed in U.S. Pat. No. 3,903,239 requires an extra processing step to meet existing product quality specifications. In contrast, the process according to the present invention does not require such an extra processing step to meet market specifications.
The present invention relates to an economical hydrometallurgical process for producing pigment grade TiO2 from titaniferous mineral ores and in particular from ilmenite ore. The ore is leached with hydrochloric acid solution in optimal conditions of temperature, pressure and concentrations to form a leachate containing titanium and iron chloride and a residue. Preferably, at least a portion and, more preferably, at least a majority of the hydrochloric acid solution is derived from recycling that is part of the process. For example, all the chloride streams may be recycled to produce gaseous hydrochloric acid via pyrohydrolysis of the iron chloride crystals and distillation of hydrochloric acid solutions. The recycled hydrochloric acid solution may be aqueous or may contain a gaseous portion.
The leachate may be filtered to separate the leachate from the residue. The leachate is cooled to a temperature sufficient to form crystals of FeCl2, which are separated from the leachate. The leachate may be subjected to a reduction step to reduce ferric iron (Fe+3) to ferrous iron (Fe+2), before crystallizing. The leachate is subjected to a first solvent extraction to form a pregnant strip solution containing titanium and ferric ions and a raffinate containing ferrous iron and other impurity ions. This pregnant strip solution is subjected to a second solvent extraction to form a second strip solution containing ferric ions and a raffinate containing titanium ions. The first strip solution may be subjected to an oxidization step before the second solvent extraction. The second raffinate contains a very pure titanium chloride solution that may be hydrolyzed into pigment grade TiO2.
Hydrolysis may be accomplished by heating and dilution of the solution. Because of the low impurity content, it is also possible to hydrolyze the solution by complete evaporation, while adding dopants that will precipitate in the bulk of the TiO2 particles and allow precise control of the characteristics of the resulting TiO2 product. This controlled total evaporation reaction may be conducted in a spray dryer. The process in which liquid solution containing titanium is sprayed into a reactor, the solution is evaporated until the titanium hydrolyzes, and the resulting hydrolyzed titanium is dried until it is substantially or completely dry will be called spray hydrolysis.
Thereafter, the recovered TiO2 may be finish processed.
The advantages of the process to produce pigment grade titanium dioxide according to the present invention include:
the use of ilmenite or other inexpensive titanium oxide ore as a raw material
the use of gaseous HCL to enhance leaching rates and completion
a succession of processing steps insuring fast leaching kinetics and good
recovery of Ti from the ore and the production of a very pure Ti chloride solution allowing hydrolysis by complete evaporation
a high quality titanium dioxide pigment product, with the potential to add well dispersed dopants and to vary the characteristics of the product over a wide range by simple changes to the operating conditions, the type, and the quantity of dopants
recovery of the iron as an oxide of possible commercial value
substantially complete regeneration of all chlorides to gaseous hydrogen chloride to be completely re-used and recycled in the leaching step.